linux-kernel - Re: [PATCH] slub/slab: fix kmemleak didn't work on some case

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Date:	Wed, 10 Jun 2015 15:45:50 +0800
From:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To:	Catalin Marinas <catalin.marinas@....com>
CC:	Christoph Lameter <cl@...ux.com>,
	"Liu, XinwuX" <xinwux.liu@...el.com>,
	"penberg@...nel.org" <penberg@...nel.org>,
	"mpm@...enic.com" <mpm@...enic.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"He, Bo" <bo.he@...el.com>, "Chen, Lin Z" <lin.z.chen@...el.com>
Subject: Re: [PATCH] slub/slab: fix kmemleak didn't work on some case

On 2015/6/9 23:03, Catalin Marinas wrote:
> On Tue, Jun 09, 2015 at 09:10:45AM +0100, Zhang, Yanmin wrote:
>> On 2015/6/8 18:13, Catalin Marinas wrote:
>>> As I replied already, I don't think this is that bad, or at least not
>>> worse than what kmemleak already does (looking at all data whether it's
>>> pointer or not).
>> It depends. As for memleak, developers prefers there are false alarms instead
>> of missing some leaked memory.
> Lots of false positives aren't that nice, you spend a lot of time
> debugging them (I've been there in the early kmemleak days). Anyway,
> your use case is not about false positives vs. negatives but just false
> negatives.
>
> My point is that there is a lot of random, pointer-like data read by
> kmemleak even without this memset (e.g. thread stacks, non-pointer data
> in kmalloc'ed structures, data/bss sections). Just doing this memset may
> reduce the chance of false negatives a bit but I don't think it would be
> noticeable.
>
> If there is some serious memory leak (lots of objects), they would
> likely show up at some point. Even if it's a one-off leak, it's possible
> that it shows up after some time (e.g. the object pointing to this
> memory block is freed).
>
>>>  It also doesn't solve the kmem_cache_alloc() case where
>>> the original object size is no longer available.
>> Such issue around kmem_cache_alloc() case happens only when the
>> caller doesn't initialize or use the full object, so the object keeps
>> old dirty data.
> The kmem_cache blocks size would be aligned to a cache line, so you
> still have some extra bytes never touched by the caller.
>
>> This patch is to resolve the redundant unused space (more than object size)
>> although the full object is used by kernel.
> So this solves only the cases where the original object size is still
> known (e.g. kmalloc). It could also be solved by telling kmemleak the
> actual object size.

Your explanation is reasonable. The patch is for debug purpose.
Maintainers can make decision based on balance.

Xinwu is a new developer in kernel community. Accepting the patch
into kernel can encourage him definitely. :)

Yanmin

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